The overall goal of this work is to find effective ways of controlling influenza. Influenza vaccines need to be updated annually and current antiviral drugs must be given within 48 hours to have effect, so there is a need for alternative antiviral targets. It has been known for a long time that sialic acid is the receptor for influenza virus. However, recent studies show that while binding to sialic acid increases the efficiency of entry, it is not essential. Infection by influenza viruses is thus more complex than has been previously considered. The hypothesis guiding the proposed work is that binding to sialic acid concentrates influenza virus at the cell surface, but that virus internalization requires interaction with a second receptor that is efficiently endocytosed, carrying the virus into the lysosome pathway to provide sufficiently low pH for membrane fusion and release of the viral genome. The goal for the next 5 years is to identify the mechanisms and molecules required for internalization, using biochemical, molecular and cellular approaches. This information will point the way to new targets for intervention. The specific aims are to answer the following questions: What is the binding affinity of recent H3N2 viruses to sialylated molecules? What variants of sialic acid are bound, and what is the influence of the other sugars in the sialylated glycans? What is the mechanism for internalizing influenza virus? Can any sialylated molecule act as a receptor for endocytosis, or is there a requirement for a more specific interaction? What is the role of neuraminidase? How well does the specificity of NA match that of the HA as determined in Aim 1? How do the functions of influenza HA and NA relate to those of Paramyxovirus hemagglutinin-neuraminidase proteins?